SU DU Carbs

[mk1coopers]

We are , always good to have alternatives to look at fitting to cars, always great service in Salisbury

[ianh1968]

Twin HS2s would provide enough air to reach 5500rpm, an 850 or 1000 could reach that on a single.
An S could reach 7000 on twin HS2s.
Twin HS6s or 45 DCOE with the correct chokes, ie 40s are for going to 7500 + with the rest of the bits to get it there.

NOT fitting them, in the belief that other setups would be superior (without having back to back tests done), would be a crime...
Once they have been calibrated, some kind of realistic conclusion could be drawn.

Fitting these carbs for any other reason is simply cosmetic
and trying to impress people with the car stationary.

... and how impressive would they look still in their box on the shelf,
in the back of a dark cupboard, in pristine condition, in 20 years time,
having never been tried?

As I said on here recently:
"If you never tried them,
and they would have been better,
you will never know".

Good Luck! I'd love to know how all this works out...
Please keep us updated!

Ian

[Jamie]

I also think its interesting, looking forward to reading how you get on.

[wantafaster1]

I'm interested too.

[steve65]

Thanks for posting Mark, a few people on here don't seem to believe me when I said that the DU6 performed well, I was the one at the rollers with it, and saw the results for myself. With a bit of fettling im sure it be even better! I am glad that you are persevering with idea, if you don't try you will never know! Its easy to stick with what you know works already, but something else may be better!
Steve.

[Cheeser]

Very good to see this, good luck with the development and will be keen to see comparitive figures after fettling! Interested to see that the manifold is running with a large bore balance pipe, does this improve a particular area of the performance? Although the manifold is relatively short I'm sure Maniflow will have created it to be just as effective as a similarly short weber manifold and could be improved upon by modifying the bulkhead ?

[minimans]

Perhaps I didn't make that clear enough. This is my mk1 racer and happily revs to 8000. With the DU carb it is leaning off after 5500 (not unexpectedly) and that is where I need to spend my time next...
I'm reasonably well versed in SU fitment/sizing etc. As I said, this is really for a bit of fun at the moment... Just thought peeps might be interested...

How about a pair? the inlet tracts would still be a long way off straight!

[su_mark]

As I said earlier, there isn't a huge amount of science going into this at the moment!

The manifold was made for me by Maniflow using some stock Weber manifold bends and cutting a new flange (as it differs from the Weber one). It is by no means the most efficient manifold, not only due to the significant 'swan neck' but also due to the bend not being quite right for the flange placement. But I just wanted to get something thrown together quickly. It was originally fabricated without a balance pipe and I added the one it has now in an effort to rid it of some pretty awful fueling issues. This has all but cured that problem, but finer tuning of the balance pipe bore size would be necessary to make things spot on.

Interestingly, on the engine dyno (using my engine), changing from this manifold to a 4.5" 'straight' Weber manifold with adaptor made little or no difference to power or torque figures.... but, again, this was not a fully exploited opportunity so further investigation needed.

Ultimately, if trying to keep the carb under the standard bonnet, without modifying the bulkhead, a cast manifold would be the way to go to get the curves as good as they could be in the limited space.

I probably shouldn't say yet but the power increase over the H4 set up (only up to 5500rpm at the moment) is a steady 6bhp at the wheels. And between 6-8 lb/ft torque. Useful stuff.

Plenty of work still to do.

A pair, on a Mini? Hmmm, might be overkill somewhat! I'm not sure if they could be split like Webers, I'd have to pay that some thought....

[ivor badger 2]

How about a pair? the inlet tracts would still be a long way off straight!

You mean like the twin HS6 kit that Special Tuning sold 45 years ago?

[mk1]

Fascinating stuff Mark!

It must be wonderful bringing these long "dead" projects back to life. I am sure everyone on here is very jealous

One quick question. Can you briefly explain how the balance pipe works & what it does, I have my own ideas, but would be interested to hear "the definitive" answer.

Thanks for your continued input on the forum.

mark F

[su_mark]

Ah, a hotly debated subject!

Here’s my take on it.

The principle of the balance pipe is to aid flow on a multi carb set up when only one carb is ‘seeing’ a bank of cylinders at a time. A good way of looking at this I stole from somewhere once before (apologies, can’t remember where!). Think of the firing order at a different point than normal, 3-4-2-1. One carb ‘sees’ 3 and 4 then the other carb ‘sees’ 2 and 1. So, with a large gap in between, the pulsations become less smooth compared to a single carb set up. The balance pipe communicates manifold vacuum to the opposite side thus enabling a flow of air to continue through the carb whilst the cylinders that carb ‘sees’ are not breathing, smoothing out flow and an otherwise stagnant period.

A single SU carb will give close to optimum mixture spread on most four cylinder engines. When fitting a pair of SU’s on a four cylinder engine it is possible to ‘tune’ the air flow (pulsations) to give correct mixture spread from full to part throttle. This is achieved by adding a balance pipe to the inlet manifold and ‘tuning’ this by changing the internal bore size of the pipe.

Interestingly, The DU carb features a full throttle weakening device designed to overcome large variations in mixture spread due to heavy full throttle pulsations, when feeding one cylinder with one carburetter choke.

[mk1]

Thanks Mark,

That makes perfect sense.

M

[Cheeser]

SU Mark,
Thanks for the explanations, one thought, would it be possible to have a static adjustable bore restrictor(bolt!) to alter the effective balace pipe bore? I'm sure Dave D at Maniflow will have already tried and discarded this idea?

[su_mark]

Very possible. I have heard of people doing this to gauge an idea of what size to go for, but I believe, given the nature of sudden 'big to small' holes and bolts protruding into passageways, it is not best practice as it can (and probably will) adversely affect the flow which needs to be as unhindered as possible.

[ivor badger 2]

Interestingly, The DU carb features a full throttle weakening device designed to overcome large variations in mixture spread due to heavy full throttle pulsations, when feeding one cylinder with one carburetter choke.

Being that the carb was probably designed for a single choke/ single cylinder setup, Coventry Climax and Jaguar?

[su_mark]

Spot on. Primarily developed in limited numbers for the FPF Coventry Climax, but used elsewhere too, Jags, Healeys and the Sabrina engined Le Mans Triumphs to name a few. Not always one choke per cylinder, but more often than not.

Don't quote me on this but I believe there was an Irish racer who put an Anglia(?) engine in the back of a Mini, fitted with a pair of DU6 carbs, back in the 60's!!

[ianh1968]

<SNIP>Primarily developed in limited numbers for the FPF Coventry Climax

... and obviously, they should NEVER have even bothered investigating putting this engine in a car,
because it was a basically a lightweight water pump for fire-engines...

(The FPF was "Fire-Pump-Featherweight", wasn't it?)

Without someone saying "I wonder if...", the Lotus story may well have turned out
completely different.

And they should never have put the 18 cylinder, opposed piston, three-cranked,
turbo-charged two stroke diesel "Deltic" engine in a locomotive either, because
it came out of a boat... Another lightweight high performance engine that was used
to stunning effect in something other than what was intended.

The conclusion here is that if you want to try something to see what happens,
GO FOR IT! We should all be encouraging innovation, not putting up barriers...

Ian

[steve65]

My thoughts too, Ian! On with the development!

[ivor badger 2]

Ah the Dark and Brave class MTBs, noted for the time spent in the dockyard, at least it saved fuel. The Deltic built by Napier, famous for the Napier Sabre engine for the Hawker Typhoon/ Tempest. A 24 cylinder sleeve valve and mechanical work of art. There's sectioned one in the RAF museum in Hendon. At the start of WW2 a Spitfire cost £5,000, the price rose during the war to £10,000. The price to Spitfire clubs never rose btw. A Napier Sabre cost £15,000 just for the engine.

Some things are a bit like the man being asked directions "well if I was going there, I wouldn't start from here"

[su_mark]

Ooh, ooh, did someone mention the Napier Sabre??

Fitted with an SU carb i'll have you know!

I've got one sitting around somewhere. Hmm, wonder if it will fit under the bonnet of the Mini......